The invention relates to a method for controlling an exhaust gas temperature, particularly in a commercial vehicle comprising a diesel engine.
The future legislation for heavy duty diesel engines requires an exhaust gas aftertreatment system for emission reduction. Both particulate filters and NOx reduction Systems needs elevated exhaust gas temperatures of about 250° C. to 350° C. for providing a reasonable efficiency in NOX conversion and soot removal, respectively. The exhaust gas temperature during motoring and light engine braking are so low that NOx reduction system will not work until it is heated up again. Regeneration of particulate filters is interrupted and NO2-based regeneration of particulate filters will not work until it is heated up again. Several methods are known in the art to increase the exhaust gas temperatures under certain driving conditions, for instance by burning additional fuel for heating up the exhaust gas temperature. In WO 2008/015406 A1 use of an exhaust throttle valve is disclosed to increase the exhaust gas temperature.
WO 2007/032714 A1 discloses a method of maintaining a high temperature in the exhaust gas aftertreatment system. The method prevents the engine from pumping cold air to the exhaust gas aftertreatment system when the vehicle is coasting by recirculating an exhaust gas flow through an exhaust gas recirculation flow duct. It is suggested to employ an exhaust brake or a variable geometry turbine for braking the engine when the fuel supply is interrupted e.g. during coasting of the vehicle
It is desirable to provide an improved method which allows to maintain an exhaust gas temperature at a level that ensures that the exhaust gas aftertreatment system can operate in an adequate temperature range. It is also desirable to provide a vehicle with an improved emission reduction.
A method is proposed for controlling an exhaust gas temperature of an engine of a vehicle during interruption of fuel supply to the engine, wherein the engine drives a driveline of the vehicle. Operation of the engine is alternating between two or more operational modes, wherein at least one of the operational modes maintains a higher exhaust temperature than the one or more other operational modes. Particularly, it is avoided to cool down components such as catalytic converters and/or particulate filters in the exhaust gas aftertreatment system when the engine does not generate enough heat.
Expediently, two different strategies can be used in the low torque area, i.e. during friction braking. In the first case the engine is moved to idle and the engine speed is changed to a lower speed. Here, using an automated mechanical transmission (AMT) is expedient. In the second case, exhaust gas recirculation can be used with unchanged engine speed. An automated mechanical transmission is not required in the latter case.
Favourably, the exhaust temperature can be established in a desired temperature range although the engine does not actively deliver heat by combustion. Catalytic converter and particulate filter can be stabilized in working temperature range. When the fuel supply is restarted, the components are ready for conversion of NOx and burning soot at once thus reducing the overall emissions of the vehicle. A heat up period for heating up the components with reduced exhaust cleaning efficiency can be avoided. It should be noted that a difference is between the temperatures of the exhaust gas aftertreatment system, particularly of catalytic converters and particulate filters, and the exhaust gas itself. According to an aspect of the invention the temperatures of these components can be maintained in a temperature range where the components are operable, i.e. where the selective reduction catalytic converter converts nitrogen oxides with sufficient efficiency and where regeneration of the particulate filter is possible. The rather large thermal masses of the catalytic converters makes them expensive and difficult to heat up at low average load on the engine.
According to a favourable method step according to an aspect of the invention, a frequency and/or a dwell time can be adjusted in at least one of the operational modes depending on a desired brake torque. Advantageously, the desired brake torque can be established even with operational modes with a lower brake torque than the desired one when alternating with operational modes with higher brake torque. Preferably, the desired braking torque can be created by alternating between motoring with EGR recirculation and a higher engine brake level. The motoring working point will not cool down the exhaust gas aftertreatment system due to extremely low mass flow. The higher engine brake level will have high enough temperature to not cool down the after treatment system. The result is an average temperature that is significantly higher than the low engine brake
When the fuel supply to the engine is interrupted, the vehicle is decelerated by engine braking by using the energy-requiring compression phase of the engine to dissipate energy and slow down the vehicle. This braking mode is also called compression braking. Large trucks use also a device called an exhaust brake to increase the effectiveness of engine braking by help of an exhaust pressure governor (EPG). In non-hybrid vehicles engine braking is active regardless of the transmission type in the driveline and is activated when the foot is lifted off the accelerator, the transmission is not in neutral, the clutch is engaged and a freewheel is not engaged. This is also called engine drag. Active use of engine braking, i.e. shifting into a lower gear, is advantageous when the driver is desirous to control speed while driving down very steep and long slopes. It can be applied before regular disk or drum brakes are applied, leaving these brakes available to make emergency stops. The desired speed is maintained by using engine braking to counteract the acceleration due to gravity when driving downhill.
According to a favourable method step according to an aspect of the invention, particularly when an engine brake torque is required, engine braking operational mode generating a first brake torque can alternate with an operational mode with exhaust gas recirculation generating a lower brake torque than the first brake torque. The exhaust gas recirculation causes only a small gas flow flowing through the exhaust gas aftertreatment system thus keeping the temperature high and avoiding to cool the catalytic converters or particulate filter with cold gas flow. Particularly the first brake torque can be established during the engine braking operational mode to a value above a desired brake torque so that the desired brake torque is established as torque averaged over the two or more operational modes. The engine braking operational mode with a higher torque level than desired can be performed first before performing the operational mode with exhaust gas recirculation, so that the temperature of the exhaust gas system is still at a high level initially in the engine braking with a high brake torque phase before the exhaust recirculation keeps circulating warm exhaust in the engine. Residual heat can be stored in the catalytic converter and the particulate filter without an unwanted cooling gas flow.
According to a favourable method step according to an aspect of the invention, particularly during idling of the engine when virtually no brake torque is required, disengaging the engine from the driveline can be performed in one of the operational modes alternating with an engine compression brake operational mode for generating a brake torque. In this mode it can be avoided to pump cool air through the exhaust gas aftertreatment system. Residual heat can be kept in the gas aftertreatment system. Particularly, throttling an air flow to an intake manifold of the engine can be performed while the engine is disengaged from the driveline. Advantageously, a pressure in the inlet manifold of the engine can be reduced in a further development of an aspect of the invention. Favourably, the gas flow through the exhaust gas aftertreatment system can be reduced, so that the exhaust gas aftertreatment system is exposed only to small amounts of gas which might probably cool the system components.
According to another aspect of the invention, a vehicle is proposed in which an exhaust gas temperature of an engine is controllable by a method according to any method feature described above. Expediently, the vehicle shows lower emissions during normal driving operation and a lower fuel consumption as burning of additional fuel for heating up the exhaust gas aftertreatment system can be avoided.